CH 14 & 15 Vision and Perception_Beams Flashcards
Image Quality Factors
1)Density 2)Contrast 3)Detail 4) Distortion
Density
Overall blackness of image (aka OD & brightness)
Contrast
Differences in adjacent density & the ability to visualize the differences in structures.
Detail
Sharpness/edges of the structures (Film/Lp/mm) (Digital- spatial rsn-pixel size & count)
Distortion
misrepresentation of object size or shape.
3 types of distortion
Shape: elongation and foreshortening. Size magnification.
Threshold Detection is:
a visual phenomenon involving the perception of extremely small or faint details.
Threshold Detection is affected by:
Anything that takes away detail, like radiographic fog (or any artifacts).
Penumbra
Blurred or unsharp.
Two main methods to control scatter:
Collimation and use of grid
Collimation
Helps reduce the production of scatter.
How does colliation help reduce scatter?
When using a smaller FOV there is 1)smaller xray field 2)less x rays 3)less compton interations 4) less scatter 5)less fog
When is a grid used?
After it’s been produced.
What is a grid?
Device that contains thin strips of lead to attenuate scatter and allow hard energies to pass through.
Beam limitation devices:
Variable rectangular, PBL/Positive beam limitation device (these 2 are the most common), Cylinder Collimator, Cone, Diaphragm Aperature.
What is a PBL?
Automatically restricts the size of the field to the size of the IR.
What happens to image when kV is inc?
Force, interations, Compton, Scatter, and Fog all increase.
Increasing part thickness does what to scatter?
Increases
What happens to scatter when atomic # is decreased?
it increases.
Umbra
Sharpness of shadow
How do we control Umbra?
Use geometric factors like OID, SID, FSS
What is Half Value Layer (HVL)?
Any absorbing material that reduces xray energy to half (50%).
Short hard energies of xrays are called?
Hardening the beam
How do we process visually?
The specialized cells of the retina, especially fovea centralis, convert the image to nervous impulses, and the optic nerve transmits the impulses to the brain for processing.
Where does human image conversion occer?
The rod and cone cells.
How do rods and cones work?
They contain photosensitive pigments that will respond to light by sending an electrical potential to specialized nerve cells.
How many cones do humans have?
7,000,000 located only at the fovea centralis
How many Rods do humans have?
120,000,000
Photopic
Daylight vision, which is controlled by cones.
Scotopic
Night vision, controlled by the rods.
Visual acuity:
sharpness or focus
Which is able to detect changes in brightness better? Rods or Cones?
Cones, reponsible for greater contrast perception.
